Asphalt Products and Processes
Pavements of asphalt compositions account for over 90 percent of the paving in the United States. Natural asphalts obtained from lake beds were utilized as early as 1874. Later, rock asphalt deposits were found in some southern and western states which were ground, placed and rolled to form pavement surfaces. However, since the early 1900's, asphalts produced in the process of refining petroleum have dominated both paving and roofing applications.
Asphalt is a dark brown to black, highly viscous material containing bitumens as the principal constituent and is found in varying proportions in most crude petroleums. The asphaltic residuum from petroleum refining, substantially freed of lighter overhead fractions, is commonly called "asphalt."
Paving asphalts are classified as asphalt cement, cutback asphalt and asphalt emulsions. Asphalt cement is of first interest here, although reference to asphalt emulsions and cutback asphalt is appropriate for perspective.
Asphalt cement is an asphalt having properties suited to road or roofing applications and specialty products. For road construction, asphalt is heated to a free-flowing consistency and mixed with aggregate heated approximately to the same temperature (usually 250.degree.-350.degree. F.) and placed on a prepared surface, compacted and cured to produce asphaltic concrete. In the long history of asphalt paving, the hot-mix process of mixing asphalt cement and aggregate has remained the process of choice as offering the most favorable balance of cost and quality. The hot-mix process brings heated, liquefied asphalt cement into contact with heated aggregate to produce a coated aggregate ready for laydown and compaction.
Asphalt cements used for paving are graded according to three distinct parameters: viscosity, viscosity after aging, and penetration. The most common grading system in the United States is based on viscosity, measured in poises at 140.degree. F. (AASHTO M-226). (AASHTO is the designation of the American Association of State Highway and Transportation Officials.) Thus, asphalt cement having a viscosity of 250 poises at 140.degree. F. carries the designation AC-2.5 and is considered a "soft" asphalt. At the other extreme, asphalt cement having a viscosity of 4,000 poises at 140.degree. F. is known as AC-40 and is considered a "hard" asphalt. In between are asphalts designated AC-5, AC-10, AC-20 and AC-30, similarly related to their respective viscosities. In addition, AC-50 has come into use in certain areas of hot climates and AC-1 has been used in colder climates. The standard asphalt grades are tabulated and discussed in "Principles of Construction of Hot-Mix Asphalt Pavements", The Asphalt Institute, Manual Series No. 22 (MS-22), January 1983, page 14.
Some western states have adopted a grading system based on viscosities after aging. This system is intended to reflect more accurately the viscosity characteristics of the pavement after it has been in place. The test simulates aging in the asphalt by accelerating oxidation of a thin film of asphalt at 140.degree. F. (AASHTO M-226). Results are reported, for example, as AR-10 for a viscosity of 1,000 poises, considered a "soft" asphalt, and AR-160 for a viscosity of 16,000 poises, considered a "hard" asphalt. This grading system is discussed in the aforementioned publication at page 15.
Asphalts may also be graded by standard penetration tests (AASHTO M-20). In these tests, the distance a standard needle bearing a specific load penetrates the asphalt in a given time at 77.degree. F. indicates the hardness or softness of the asphalt. This test is discussed in the aforementioned publication at page 16.
For roofing application, asphalt cement is used in the construction of built-up roofs, shingles and saturants in asphalt roll-roofing. Asphalt cement used in built-up roofs is graded by softening point according to ASTM D312. (ASTM is the designation of the American Society for Testing Materials.) A Type I asphalt, which has a low softening point, is considered a soft asphalt. Type IV roofing asphalt has a high softening point and is considered a hard asphalt. These and intermediate grades are based on the susceptibility of the asphalt to flow at stated roof temperatures and slopes. Built-up roofs are constructed by rolling out asphalt-saturated felts, followed by mopping asphalt cement thereon. This process is repeated several times to produce a waterproofed, built-up roof.
There are other specialty applications for asphalt cement including, for example, joint and crack fillers, recycling agents and waterproofing and dampproofing, which have various requirements according to the intended use.
Cutback asphalt is used where the asphalt is desired to be liquefied at temperatures lower than those normally employed with asphalt cement or without emulsifying (see below). Cutbacks are commonly applied as spray applications. They are prepared by dissolving asphalt in a petroleum solvent, such as naphtha, kerosene or fuel oil. Both spray and cold-mix applications involving cutbacks raise environmental and safety problems through release of the solvent to the atmosphere. Also, in the energy crisis of the 1970's, the use of petroleum solvents for this purpose was contrary to conservation measures then imposed, which has resulted in a substantial reduction in cutback usage today.
Asphalt emulsions normally employ no solvents for their preparation, although cutback may be used as the asphalt component (these are normally water-in-oil emulsions). The asphalt flux is liquefied by heating, and globules of asphalt are dispersed in water and milled with a surfactant to produce a stable oil-in-water emulsion. Asphalt emulsions can be one of several types, which include anionic, cationic and nonionic, depending on the surfactant used to make the emulsion. Emulsions are used in the sealing of existing roads by applying a thin film of the asphalt emulsion to the road surface, followed by a covering of aggregate to provide a waterproof road. Asphalt emulsions can also be used for mixing with aggregate in place on the roadbed or, through a cold pug-mill process, with aggregate which is then distributed by a aver on the road. Emulsions are usually associated with cold-mix processes; when used as hot-mix, lower temperatures are usually employed as compared with conventional hot-mix processes.
Asphalt emulsions can be used in the hot-mix process to produce asphaltic concrete, but inherent manufacturing difficulties have conferred general preference on utilizing asphalt cement. Some of these problems associated with asphalt emulsions in the hot-mix process are discussed below.
In batch hot-mix plants, venting of the water vapor released on heating the emulsion (normally containing about 30% water by weight) sometimes occurs with explosive force where the aggregate is brought to a relatively high temperature, creating safety and environmental problems. In the continuous drum hot-mix plants, the short mixing time is sometimes insufficient to afford adequate release of water. In both hot-mix manufacturing processes, there is a substantial additional amount of energy required to evaporate the water contained in the emulsion. These oil-in-water emulsions are subject to freezing if stored at sufficiently low temperatures, with consequent premature breaking of the emulsion. Should the emulsions for some reason be overheated, water can be prematurely lost and the emulsion inverted, causing potentially serious problems in handling and resulting in the loss of the use of the product.
Most important from the standpoint of quality is the need to remove water as quickly and completely as possible from the emulsion residue adhering to the aggregate. The water phase of the emulsion contributes inevitably to a high water content in the asphaltic concrete at laydown, and the rate of subsequent evaporation can be influenced by environmental conditions. Thus, there is uncertainty in both the rate and extent of drying in the curing stage of asphaltic concrete laid from asphalt emulsions, with accompanying prospects for variability in important characteristics at any given point in the curing process.
Asphalt emulsions which have been used in the hot-mix process include a class of anionic emulsions called "High float" emulsions. The preparation of these emulsions has long followed established procedures in which the emulsion is stabilized by in situ saponification of organic acids, usually present as tall oil. An asphalt with improved residue properties is produced after the removal of water in the hot-mix process.
For example, U.S. Pat. No. 2,855,319 describes an emulsion in which tall oil is saponified by sodium hydroxide to yield a tall oil soap which serves as the emulsifying agent that is said to confer improved properties on the emulsion residue of the cured asphaltic concrete. U.S. Pat. No. 3,904,428 similarly describes an asphalt emulsion in which, for example, tall oil saponified with sodium hydroxide in the presence of substantial amounts of water is milled with the asphalt cement in a particular temperature range to produce a viscous jelly-like mass containing higher than usual amounts of asphalt. The higher asphalt content is said to lessen the tendency of the asphalt to drain from the wet aggregate and yields a more complete coating.
U.S. Pat. No. 4,433,084 describes high-float emulsion processes in which tall oil is first mixed with asphalt pretreated with various modifiers that affect the properties of the asphalt but do not influence the breaking of the emulsion. Also disclosed is a process in which the emulsifier comprising, for example, tall oil reacted with caustic in water solution, is blended with the asphalt. Ratios of emulsifier components may be varied to accommodate varying compositions of asphalt.
A publication of the Tall Oil Products Division of the Pulp Chemicals Associations, "Tall Oil And Its Uses" (F. W. Dodge Company, 1965), emphasizes the importance of surfactants in the emulsion to displace water on the aggregate and to facilitate binding of the asphalt cement thereto. For this purpose, it describes the use of tall oil fatty acids as emulsifying agents in fluidizing asphalt for road applications.
A general review of hot-mix and cold-mix paving processes is found in "Highway Engineering," Wright & Paquette, 4th Edition (John Wiley & Sons, 1979). A more current review of the hot-mix process appears in "Principles of Construction of Hot-Mix Asphalt Pavements", The Asphalt Institute, Manual Series No. 22 (MS-22), January 1983, to which earlier reference was made. For a review of cold-mix processes using asphalt emulsion, see "A Basic Asphalt Emulsion Manual," The Asphalt Institute, Manual Series No. 19 (MS-19), March 1979.
The saponification reaction has been applied in the solidification of normally liquid hydrocarbons, such as gasolines, to facilitate their safe handling and use. For example, U.S. Pat. No. 2,385,817 discloses the solidification of "normally liquid hydrocarbons" by the formation in situ of metallic soap obtained from the saponification of a mixture of stearic acid and rosin with sodium hydroxide and a small amount of anhydrous methyl alcohol. The alcohol is said to "expedite" the reaction. The "liquid hydrocarbons" are gasolines and other petroleum distillates that are readily flammable and are intended for use as combustible fuels. As such, they are cuts considerably lighter in the petroleum refining process than the asphaltic residuum.
Similarly, soap greases, likewise based on lighter petroleum cuts, have been described, for example, by Lockhart, American Lubricants (Chemical Publishing Company, 1927), page 163 et seq. and in U.S. Pat. No. 3,098,823. It has been recognized, not surprisingly, that water is an undesirable ingredient in a grease. For example, in U.S. Pat. No. 2,394,907, a grease is prepared by suspending sodium hydroxide in "a nonreactive liquid medium", such as mineral oil, milling the sodium hydroxide therein and saponifying a fatty acid in the absence of added water. Heating the mixture to a "saponification temperature" is said to initiate the reaction, producing undesired water as a by-product, which then must be removed.
In U.S. Pat. No. 2,888,402, a similar reaction is described but one utilizing a metal hydroxide having water of hydration which is released on heating nd which, it may be supposed, initiates the saponification reaction. Lithium hydroxide, specifically alluded to as the source of water, initiates a first stage saponification, followed by a second stage in which other metal hydroxides are employed.
Despite the long history and extensive use of greases in which organogels were produced by in situ saponification, the arts utilizing asphalts never translated and adopted grease technology to achieve the substantial benefits of gel formation in asphalt materials. Instead, application of asphalt to road, roofing construction and specialty asphalt applications have remained until the present invention the technological province of conventional asphalt cement and, to a lesser extent, of cutback and emulsion processes.